In recent years telescope camera systems have been developed where the telescope consists of an optical system mounted to provide low friction rotation about two mutually perpendicular axes. The optical system is motor driven and permits adjustment of the telescope orientation, to prevent the star from appearing to be in continuous motion and drifting out of the field of view. In addition, the telescope camera systems include a camera with a shutter mechanism and image capturing material. Furthermore, no additional lens is needed in the camera because the telescope acts as the lens.
Conventional systems for star imaging use charge-coupled device imagers (CCD) or active pixel sensors as imaging sensors. CCD imagers usually have limited capacity due to size and weight as well as being susceptible to radiation damage. Furthermore, in a CCD device, an electrical charge is transferred from within the device to an area where the charge may be digitized. Specifically, the CCD imager includes a photoactive region (e.g., a silicon layer) and a transmission region. An image is projected through a lens onto a capacitor array (e.g., the photoactive region) causing each capacitor to accumulate an electric charge proportional to the light intensity. In addition, a control circuit causes each capacitor to transfer the electric charge to create a sequence of voltages which may be digitized and stored in a memory.
However, due to the sensitivity of the CCD imager, thermal noise and cosmic rays may alter the pixel in the CCD array, thereby causing shifts or hot spots in the imaging and degrading the star image. To avoid the disturbances caused by thermal noise, multiple exposures may be taken with the CCD shutter both closed and open which creates a baseline. For example, an average of the images taken with the shutter closed is taken to lower thermal noise and the average is subtracted from an open-shutter image to remove image defects.
Moreover, a CCD imager has also been developed in connection with an image intensifier mounted in front of the CCD (ICCD) imager wherein photons are accelerated, as shown in FIG. 1. Specifically, photons hit a photocathode 105, thereby generating photoelectrons that are accelerated towards a micro-channel plate 115 by an electrical control voltage applied between the photocathode and the micro-channel plate. The photoelectrons are then accelerated toward a phosphor screen 110 that converts the photoelectrons back to photons that are guided to the CCD by the optical system. In addition, the amplifiers in the ICCD are mounted externally to increase the photon-generated charge above the read noise of the imager. Furthermore, the advantage of ICCDs is the gateability wherein the voltage between the photocathode and the micro-channel plate is reversed and the image intensifier may operate as a fast optical switch to collect more pixel charge.
In addition, an electron-bombarded semiconductor gain process has been developed using a photomultiplier tube that includes a photocathode, electrodes, and a collection anode having a semiconductor diode. Although the electron-bombarded CCD is not widely used as a detector for low light camera systems, the electron-bombarded semiconductor has been applied to the ICCD to prevent image degradation. Furthermore, to form the electron bombarded CCD, a back illuminated CCD has been used as an anode in focus with the photocathode. The backside illumination system includes a silicon wafer about 750 microns thick that contains a thin layer of light sensitive pixels on top of which several additional layers are placed to collect light emitted from an object such as a star. However, the electron-bombarded CCD exhibits limited gain adjustment range and similar to the ICCD, produces images having low resolution.
Alternatively, many star imagers apply a complementary metal-oxide-semiconductor (CMOS) imager to an active pixel sensor device. A CMOS imager uses a combination of p-type and n-type MOS field effect transistors to implement logic gates. A p-type transistor exists when a doped semiconductor contains excess holes and an n-type transistor exists when a doped semiconductor contains excess free electrons. In addition, the CMOS imager chip integrates amplifiers and analog to digital converters in the device thereby lowering the cost of a camera system to which the imager is applied. The CMOS imager may also integrate other functions such as timing logic, exposure control, shuttering, white balance, gain adjustment, and initial image processing algorithms.
Furthermore, each pixel in a CMOS imager chip contains conversion electronics that may read the accumulated charge as opposed to a whole charge being carried across the chip like the CCD imager. In other words, each pixel in the CMOS incorporates both the photodiode and a readout amplifier, wherein the charge accumulated by the photodiode may be converted into an amplified voltage inside the pixel and then transferred to the rows and columns of the chip. The structure of a CMOS imager allows for high noise immunity and low static power consumption since it does not rely on charge transfer, thereby preventing potential image shifts. Unlike the CCD imager, a CMOS imager only draws significant power while the transistors are switching on and off and thereby producing less waste heat.
Active pixel sensors are imaging sensors that include an amplifier in each pixel configured to increase the sensitivity of the sensor to light rays, such as light rays from a star. Thus, imaging of low light objects may be improved when an amplifier is added to the optical system as the field of view is made narrower and sensitivity of the sensor increases. Additionally, the star cameras applying active pixel sensors are generally smaller in size than a CCD imager that allows the sensor to contain more pixels, thereby improving the sensitivity of the sensor. FIG. 2 illustrates an exemplary view of an active pixel sensor device according to the prior art. As shown in FIG. 2, the active pixel sensor device includes an imager chip (the CMOS imager) 210 that includes an array of photodiodes 215 operating as optical pixels. Specifically, each unit pixel includes the photodiode in which electrons are generated by incident light.
Moreover, for star imaging, the number of electrons produced is a function of the wavelength and intensity of the light striking the semiconductor. Additionally, sensitivity is determined by the maximum charge that can be accumulated by the pixel and the ability of a device to accumulate the charge in a confined region without spillover. Thus, to observe stars outside a visible light spectrum, the sensitivity of a sensor must be adjusted as compared to the sensitivity required to observe stars in visible light.
Furthermore, low light imaging has recently become very important in observation of the universe as astronomers seek to further understand the development of stars and galaxies. In addition, star imaging has been challenging due to the universe emitting light across the electromagnetic spectrum, much of which does not reach the Earth. The atmosphere's ozone layer blocks out various types of radiation made up of a stream of photons including ultraviolet rays and far infrared rays while allowing most of the near infrared radiation to pass through. The various types of radiation of the electromagnetic spectrum varies according to the amount of energy that the photons contain. Furthermore, the atmosphere allows visible light, radio waves (containing the least energy), and certain wavelengths within the infrared region to reach the Earth. Thus, ground based observation stations may observe stars within the visible light spectrum which is within a wavelength region of about 390-700 nanometers. However, not all stars may be observed from the Earth because they are outside the visible wavelength spectrum. In addition, to observe the universe, astronomers have relied on space shuttles and satellites in Earth orbit having telescopes to gather information regarding photon energy that does not reach the Earth.
Many regions of light cannot be viewed from optical telescopes because the objects are embedded in dense regions of gas and dust. Additionally, stars are theorized to be formed from collapsing gas and dust clouds and, as the clouds collapse, the density and temperature of the cloud increases. A new star forms at the center of the cloud where the density and temperature are the greatest. Since the new star is embedded in the cloud of gas and dust, it is difficult to observe or image the star in visible light. Infrared radiation has the ability to pass through dusty regions of space without being scattered. Thus, stars hidden by gas and dust in the infrared region may be studied and imaged. Furthermore, infrared radiation is detected from objects that emit heat. The infrared radiation ranges from 1 to 300 micrometers in wavelength and much of the infrared radiation is absorbed by water vapor in the Earth's atmosphere. In addition, with infrared sensitivity sensors, stars that are still in the formation process may be observed because they emit infrared radiation before the star may be viewed in visible light.
Even though many stars may be considered substantially cool and emit much of their electromagnetic radiation in the visible spectrum, star observation and imaging may be improved by providing an imaging sensor sensitive to ultraviolet radiation between about 10-320 nanometers or shorter. Light at these wavelengths is absorbed by the Earth's atmosphere, therefore the observation must be performed from the upper atmosphere or from space. The ultraviolet spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the color violet. The photons also have higher energy than the photons in the visible light spectrum, thus the ultraviolet light has shorter wavelengths than that of the visible light. The higher energies of the ultraviolet spectrum are absorbed by the gases in the air and have a substantially short path length through the air. In addition, ultraviolet spectrum measurements are used to distinguish chemical compositions, densities, and temperatures of stars.